The present invention relates generally to motor speed control circuits. More particularly, the invention relates to a variable speed control circuit for use with direct current motors. The invention is thus applicable to DC powered and battery powered tools, appliances and other products employing DC motors.
The popularity of DC operated products such as hand-held power tools, appliances and the like has risen dramatically in recent years, owing in part to the improvements made in battery pack technology. DC operated products offer a great deal of convenience, since they can be used practically anywhere, without extension cords. Such products can also be readily adapted to work from an automotive power supply, adding even more versatility.
To enhance their usefulness, many products are equipped with some form of motor speed control circuit which allows the user to select the appropriate motor speed for the particular application. Conventional motor speed control circuits have proven generally effective in AC powered products but leave much to be desired when applied to DC powered products.
The present invention provides a much improved motor speed control circuit for providing variable speed with feedback in DC powered products. The control circuit switches the voltage to the motor off and on at a preset period to vary the motor speed. By increasing the on time duration and reducing the off time duration, the motor speed will increase. In a like fashion, if the on time is reduced and the off time increased, the motor speed will decrease. The invention thus can employ pulse width modulation. The control circuit which switches motor current on and off is interposed in the ground side or low side of the motor, rather than in the high side between the motor and the power source.
In hand-held products the switch position can be made to correspond to a desired speed. During the motor off time, the motor generates a back EMF voltage which is directly proportional to the motor speed. The circuit detects this voltage and compares it with a set voltage which corresponds to the switch position. If the motor back EMF voltage is low in relation to the switch position, the circuit increases the pulse width duty cycle in order to increase motor speed and, in turn, increase the back EMF voltage to balance the circuit.
The invention employs indirect sensing of the back EMF and controls the back EMF by closed loop control. In one embodiment, a capacitor coupled to the ground side of the motor (i.e., downstream of the power supply) accumulates a charge induced by the back EMF. The charge develops a voltage in a 1-.notgreaterthan. relationship to the average back EMF. The closed loop control may be either constant frequency variable on/off, or variable frequency constant on time control. The invention employs a charge pump circuit which permits operation from low battery voltages and which operates the motor controlling power transistor in an optimum fashion.
The invention enjoys a substantially reduced hardware cost by eliminating many expensive components. The circuit is a universal control which can be used on a wide variety of DC powered products. The control circuit offers a wide range of speed control and works very well in battery powered or DC powered applications.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.